In my testing, inserting a different image, in the the same cell, simply stacks the new image over the top of the old image.
sheet.insertImage('http://somewhere.com/images/image1.png',col,row);
sheet.insertImage('http://somewhere.com/images/image2.png',col,row);
I had hoped to effectively 'remove' image1 by substituting a blank(transparent) image2. One can cover up image1 with some other opaque image. ('mask', 'overlay', 'overlap' 'conceal', 'hide')
overlapped images
In my experience, theCell.clearContent() does not affect the image but will clear other cell contents.